1. Field of the Invention
The present invention relates generally to telecommunications, and more particularly to a technique for evaluating and expressing availability of a long haul network.
2. Related Art
Among providers of telecommunication network services, the term "availability" refers to the portion of time that a given customer will be able to use a particular service or connection. Availability is measured in errored seconds, severely errored seconds, and unavailable seconds. According to ITU Recommendation G.826, an errored second (ES) represents one or more errors found in a set of consecutive bits (known as a block) being transmitted over the network within the time frame of a second. A severely errored block would contain 30% or more errored blocks within a one-second period. Unavailable seconds are the amount of seconds that the network is unavailable to the user, such as when the network is undergoing repair. Availability is a bi-directional connectivity. It can be measured electronically or from the time a network connection is undergoing repair by a craftsman.
The standard approaches for evaluating and expressing availability of an end-to-end path in a communications network, namely ITU recommendations M2100 and G.826, are solely based upon reliability estimates of the facilities that make up the path. These approaches do not include the substantial impact on availability caused by actions of network personnel in the course of maintaining the network facilities.
A substantial error is introduced as negotiators extrapolate short-haul availability figures to estimate long-haul values. This is commonly done in estimating end-to-end availability for trans-oceanic international connections. Unfortunately, such a connection usually comprises a series of communications links that vary in type, length, reliability, nationality, and ownership. Many times, the availability figures that are commonly accepted for, say, an undersea cable are arbitrarily decided by committee debates rather than empirically measured. The result is that an estimated availability including this type of communications link is usually many times worse than is actually experienced. For example, a 7500-km end-to-end intercontinental path is projected by M2100 to suffer at least 138 errored seconds every 24 hours. Conciliatory negotiations might place this number even higher for lack of better information. This results in poor expectations of performance and very lax maintenance action limits--especially considering that actual observed performance is on the order of 1 or 2 errored seconds per day. Thus, a simple extrapolation to estimate a long-haul performance is convenient, but inadequate.
What is needed is a means for characterizing actual availability for each portion of a long-haul path and then combining these "modular availabilities" into a composite end-to-end availability figure that is realistic. Further, what is needed is a system and method that can assemble a composite availability projection from a mixture of such empirically-derived availabilities for some segments and M2100-compliant estimates for other segments. For some segments, the latter figure may be decidedly adequate.